This invention relates generally to color imaging and more particularly to a method and apparatus for producing a large gamut of colors using tri-level, highlight color imaging.
In the practice of conventional xerography, it is the general procedure to form electrostatic latent images on a xerographic surface by first uniformly charging a charge retentive surface such as a photoreceptor. Only the imaging area of the photoreceptor is uniformly charged. The image area does not extend across the entire width of the photoreceptor. Accordingly, the edges of the photoreceptor are not charged. The charged area is selectively dissipated in accordance with a pattern of activating radiation corresponding to original images. The selective dissipation of the charge leaves a latent charge pattern on the imaging surface corresponding to the areas not exposed by radiation.
This charge pattern is made visible by developing it with toner by passing the photoreceptor past a single developer housing. The toner is generally a colored powder which adheres to the charge pattern by electrostatic attraction. The developed image is then fixed to the imaging surface or is transferred to a receiving substrate such as plain paper to which it is fixed by suitable fusing techniques.
In tri-level, highlight color imaging, unlike conventional xerography, the image area contains three voltage levels which correspond to two image areas and to a background voltage area. One of the image areas corresponds to non-discharged (i.e. charged) areas of the photoreceptor while the other image areas correspond to discharged areas of the photoreceptor.
The concept of tri-level, highlight color xerography is described in U.S. Pat. No. 4,078,929 issued in the name of Gundlach. The patent to Gundlach teaches the use of tri-level xerography as a means to achieve single-pass highlight color imaging. As disclosed therein the charge pattern is developed with toner particles of first and second colors. The toner particles of one of the colors are positively charged and the toner particles of the other color are negatively charged. In one embodiment, the toner particles are supplied by a developer which comprises a mixture of triboelectrically relatively positive and relatively negative carrier beads. The carrier beads support, respectively, the relatively negative and relatively positive toner particles. Such a developer is generally supplied to the charge pattern by cascading it across the imaging surface supporting the change pattern. In another embodiment, the toner particles are presented to the charge pattern by a pair of magnetic brushes. Each brush supplies a toner of one color and one charge. In yet another embodiment, the development systems are biased to about the background voltage. Such biasing results in a developed image of improved color sharpness.
In highlight color xerography as taught by Gundlach, the xerographic contrast on the charge retentive surface or photoreceptor is divided three, rather than two, ways as is the case in conventional xerography. The photoreceptor is charged, typically to 900 v. It is exposed imagewise, such that one image corresponding to charged image areas (which are subsequently developed by charged-area development, i.e. CAD) stays at the full photoreceptor potential (V.sub.cad or V.sub.ddp, shown in FIG. 1a). The other image is exposed to discharge the photoreceptor to its residual potential, i.e.V.sub.dad or V.sub.c (typically 100 v) which corresponds to discharged area images that are subsequently developed by discharged-area development (DAD) and the background areas exposed such as to reduce the photoreceptor potential to halfway between the V.sub.cad and V.sub.dad potentials, (typically 500 v) and is referred to as V.sub.white or V.sub.w. The CAD developer is typically biased about 100 v (V.sub.bb, shown in FIG. 1b) closer to V.sub.cad than V.sub.white (about 600 v), and the DAD developer system is biased about 100 v (V.sub.cb, shown in FIG. 1b) closer to V.sub.dad than V.sub.white (about 400 v).
Currently, a process known as the four-color separation process is very widely used in the printed reproduction of colored pictures, transparencies and the like. The four-color separation process is generally responsible for all of the high-quality colored reproductions in magazines and books, and is also used for some newspaper work as well.
In the most common version of this process, the original print or transparency is photographed through different filters to produce different individual films which correspond to the basic colors of the four-color separation process: yellow, magenta, cyan and black. The filters utilized to extract the first three of these colors from the original have tints which are the complementary colors of the colors being drawn out. Thus, a green filter is used to pick out the magenta, a blue filter is used to pick out the yellow, and a red filter is used to pick out the cyan. A combination of all filters is ideally utilized to pick out the black, although in some processes the black film is made by photographing the original in black-and-white film, without any filter.
Some "fine tuning" or adjustment of the intensities of the various colors in the process is made by selecting exposure times and development times. Also, in some instances the final inks used to print the final reproduction can be varied and selected to attain certain effects.
From the four pieces of film produced through this process, printing plates are made, these being subsequently attached to plate cylinders in a typical printing machine, which is then able to print the reproduction using the process colors; yellow, magenta, cyan and black.
Generally speaking, the printing industry is of the view that proper reproduction of any photograph or the like requires a four-color separation process of the kind just described, utilizing the process ink colors.
However, in certain branches of the printing industry, particularly in newspapers, shopping bags, the Yellow Pages and advertising flyers, the full four-color separation process represents a considerable expense, since it requires the material to be passed through four printing stations, in order to receive the four colors. This in turn requires the production of four plates, and the time required to mount them, adjust the components, etc. All of this represents a substantial cost factor which, for obvious reasons, it would be of advantage to reduce.
There is two-ink process forming part of the prior art, known as duotone. In one version of this process, often called "Fake" duotone, a black and white original (for example a photograph) is first photographed on "Ortho" film through a contact screen to give a screen film. "Ortho" film is a high contrast film which is not sensitive to the red region of the spectrum. Then, the same screen is rotated through an angle of 30 degrees and another screen film is taken, substantially identical to the first, also on "Ortho" film. The two films are developed to different densities, then are used to make plates which are run in two colors, for example, red and black.
In another version of duotone, often called "Real" duotone, the original art is already in two colors, for example, red and black. The first step is to shoot "Ortho" film through a gray contact screen without any filter. Both the red and the black will be seen by the film, and the result will be a film in which the red and black are both picked up as black. Then, a panchromatic film is exposed through a red filter and a grey contact screen. In this arrangement, the film sees only black. The two films are then used to make plates which print red and black, respectively.
It is also known, particularly in the food advertising area where blue colors are rare, to do a three-color separation using the standard filters to obtain yellow, magenta and cyan, and then to print the image using yellow ink, red ink and black ink. In other words, the plate made from the cyan film prints in black ink.
It is further known to mix various colors to produce various other colors. For example, it is known to print a dot matrix of black superimposed on a dot matrix of yellow in order to produce various shades of green. It is also known that red and yellow will combine in the same way to produce orange. It is further known that yellow and cyan will combine to produce various shades of green.
Disclosed in U.S. Pat. No. 4,554,241 granted to Wallace Edwards on Nov. 19, 1985 is a process for creating strikingly realistic reproductions of an original utilizing only two printing plates inked with only two different colors. However, the process of making these plates does not involve simply one of the known parts of the standard four-color separation, nor does it utilize process inks.
By way of explaining this, it should be understood that, if a four-color separation were made to produce four plates intended to print yellow, magenta, cyan and black, and then if only the yellow and red were printed, or only the yellow, blue and black, or any other combination which was not the full combination of four colors, the resulting print would be clearly and definitely unbalanced, and anyone viewing the print would immediately see the unbalanced nature of the colors. The print would appear "too reddish" or "too far into the blue region", or blotchy. The aim of the process described in the '241 patent is to remove the unbalanced nature of a printed reproduction made with only two impressions, and thus two inks.
Simply stated, the process disclosed in the '241 patent consists in making a red printer by utilizing sequentially a green filter and a blue filter, and making a printer for another color such as green, blue or black by utilizing sequentially a red filter and a blue filter. More particularly, device described therein provides a method of printing on a sheet member a realistic reproduction of a colored original, utilizing a minimum of two different superimposed impressions, each with a different coloring medium, comprising:
(a) providing a colored original, PA1 (b) creating a first printing plate intended to print a non-process red color, by PA1 (c) creating a second printing plate intended to print a second color different from that printed by said first plate, by PA1 (d) providing a sheet member to receive two superimposed impressions, and PA1 (e) using said first and second printing plates to print said red color and said different color, respectively, as the said superimposed impressions on said sheet member.
(1) making a green filter exposure of the original on a first means for recording a first optical image, PA2 (2) making a blue filter exposure of the original on said first means, steps (1) and (2) being carried out sequentially in any order, PA2 (3) making a red filter exposure of the original on a second means for recording a second optical image, PA2 (4) making a blue filter exposure of the original on said second means, steps (3) and (4) being carried out sequentially in any order, and steps (b) and (c) being carried out in any order,
The invention of the '241 patent as stated therein, is applicable to the copying industry where it is well understood that the copying process involves the establishment of a latent electrostatic image on a drum or plate constituting a photoconductive surface, following which a colored "toner" is applied to the image-containing portions of the photoconductive surface, the electrical attraction causing the toner to remain in certain areas and be removed from others, following which the photoconductive surface with the toner is applied against a sheet of paper which picks up the toner as an image. The essence of the '241 invention can be applied to the copying industry, by arranging to have the photoconductive surface exposed through not one but two filters for each of the printings. These filters ideally would be used sequentially to build up an electrostatic image which is a composite of the images which would normally be obtained through the two different colored filters. Then this composite electrostatic image is contacted by the appropriate colored toner, and the same is printed on the paper sheet.
In electronic printing on a raster device, the imagable area of the substrate is subdivided into a fine pattern of dots called pixels. The pixel is the smallest area over which one can control the placement of the colorant. The marking device can color or not color each pixel. A computer is used to instruct the marking device as to which pixels to color in order to create the desired image. The pixels are scanned in a fixed order or raster. For each pixel the computer generates a binary value indicating color or no color. An electronic subsystem converts the binary values into the control signals for the marking device. On a black-and-white printer, intermediate shades of gray may be produced by printing a rapidly alternating pattern of black and white pixels. This pattern, when viewed from a distance, has the appearance of gray. Different patterns yield different shades of gray depending on the ratio of black to white pixels. The computer may store a set of patterns which can be used for producing gray shades. The patterns are often saved as a collection of binary values indicating the black/white coloring for a small rectangular area of the image surface. The pattern is then replicated as needed to cover the entire image surface. One might alternatively store in the computer an algorithm for generating the pattern, rather than the pattern itself. In this case the pattern must be synthesized whenever needed. Each pattern is referred to as an "ink" since it results in a different shade of gray. (This is not an actual colorant of toner, but only a pattern within the computer.)
For multi-color printers, the computer must provide a raster of pixel values for each of the colorants. Various shades, hues, and tints are produced by combinations of pixel patterns for the various colorants. The design of the patterns depends fundamentally on whether the marking technology preserves registration between colors. If the raster of pixels for one colorant can move relative to the raster of pixels for another colorant, from one image to the next, then sometimes the pixels for different colors will overlap and sometimes they will not. To avoid moire effects and color shifts, the patterns are designed to distribute and smooth the overlap. This is usually done by a rotation of the relative axes of the patterns for the colorants.
In technologies where the registration between colors is precise, a different approach may be taken. In this case each pixel may be assigned a color. Tri-level electrophotography is just such a technology; in a single pass of the laser beam each pixel is set to either black, highlight color, or substrate color. For a device where registration is maintained one can create patterns (or "inks") in which each pixel is assigned one of the possible primary colors. This results in many more potential patterns than could be formed from strictly binary pixel values. For this approach one needs a scheme for generating the patterns, and if they are to actually be stored, one would like a representation which does not require the large amounts of memory which would be needed for explicit description of every pattern. The invention provides this for the case of two colorants.
U.S. Pat. No. 4,903,048 granted to Steven J. Harrington on Feb. 20, 1990 relates to simulated color imaging using gray level patterns produced from two differently colored materials by employing fine patterns of dots. The dots blend with the background and yield a gray or colored appearance when seen from a distance. The imaging process utilizes ink pattern designs in conjunction with registered two-color imaging to thereby form simulated color images. Digital information representing two sets of gray-level producing patterns, set A for color A and set B for color B, is electronically stored in computer memory. The patterns in set B are complementary to those of set A. An apparent or simulated color image is produced by overlaying, combining or juxtapositioning an pattern from set A with a complementary pattern from set B, the combined image being subsequently rendered visible using two different colorants. A gray level pattern is produced for each elemental area of an original image.